This invention relates to an improved method and apparatus for the removal of water from the well-bore of gas wells so as to reduce the hydro static head associated with the produced water and to thus unload the well and permit more efficient production of gas.
Numerous gas wells in Alberta, Canada, and throughout the world, produce a minimum of natural gas because of problems associated with water produced with the gas, which accumulates at bottom-hole, and by virtue of its weight provides a hydrostatic back pressure which partially or entirely defeats the ability of formation gas pressure to move well gas to the surface for collection.
There are a number of methods in use to remove water from the well-bore of a producing gas well when the column height of water in the bore produces a hydrostatic pressure greater than the pressure of the gas from formation sufficient to impair production.
The gas in those situations has typically been produced out of a smaller diameter tubing string inserted into the well because the smaller cross-section of the tubing produces higher velocities of gas flow at formation pressures which it is hoped will carry the water out of the well-bore during production and thus xe2x80x9cunloadxe2x80x9d the well.
As a reference, the article xe2x80x9cA Practical Approach to Removing Gas Well Liquidsxe2x80x9d written by E. J. Hutlas and W. R. Granberry (published August 1972 in Journal of Petroleum Technology) discusses the history of methods of removing unwanted accumulations of liquids from gas wells. The article states that the best methods for removing liquid from producing gas wells are pumping units, (for shallow fields having very low pressure), liquid diverters, intermitters and gas lifts (for deeper higher-pressure fields), and inserted tubing strings for wells where severe formation damage could result from stopping operations (e.g. well shock).
U.S. Pat. No. 4,265,312 to Thein, METHOD FOR DEVELOPING WATER WELLS, issued May 5, 1981, pertains to a method of developing a water well, but is instructive in that it deals with the introduction of a gas-lift water pump into a well-bore, the pump""s action being provided by introduction from surface of high-pressure gas (air, in that case) in an inner tube deployed such that its bottom end is above the bottom end of an outer tube in which the inner tube is centered, and the bottom of both of which tubes are deployed submersed in the well-bore""s accumulated water. The pressured gas escapes from the bottom of the inner tube and is collected and exits the well-bore, rising up through the annulus between the pressurized tubing and the second, outer tube, taking with it entrained water from the wellbore (within which the tubes were deployed).
In the Thein patent, the purpose was to provide a gas-lift pumping means with some agitation at the well-bore""s bottom end to remove settled solids and improve water flow within a water well. The apparatus is a good example of a form of gas-lift pump (concentrically deployed tubing strings, providing pressurized gas from surface to the inner tube, permitting gas and entrained liquids to exit using the annulus between inner and outer tube as a discharge path).
There are two related prior art downhole pump systems, U.S. Pat. Nos. 3,894,583 and 3,894,814 to Morgan, both issued Jul. 15, 1975, titled ARTIFICIAL LIFT FOR OIL WELLS, which describe a two-chamber down-hole pump apparatus to be removably inserted into an oil well""s casing to pump oil from an oil well where production pressure is insufficient to provide lift to surface.
Morgan""s ""583 patent describes an apparatus and system where the well""s casing is isolated by a packing from the production zone and from the pump at bottom-hole to provide a storage tank for compressed air, an accumulator below the packing to collect oil from formation via a check valve which is vented to surface, said vent line being periodically pressurized by connecting tubing and valves to the casing""s stored pressurized air to force the collected oil from the accumulator up a secondary tubing string to surface. A piston, pig or swab is deployed below the oil in the production tubing string. The vent/pressure tubing is deployed adjacent/concentrically to said production tubing. Crossover conduits between production tubing/venting tubing are used to switch annulus mid-bore. An electric switch/ball float is deployed in the lower well-bore to sense the accumulator""s state (full or not), and to energize an air pump top-hole and electrically switched valves to pressurize the vent tubing.
Morgan""s ""814 patent describes and claims the same system and apparatus, absent the use of the upper casing as a pressure tank and requiring an electrically actuated 3-way valve and down-hole sensor.
Morgan""s ""583 patent also describes an embodiment where the accumulator means is formed by a chamber comprised of the well""s casing below the packer isolating the upper casing""s compressed air storage area from production, and also below a second packer below the perforations in the casing to formation, with a check valve from the thereby isolated production zone of the casing into the accumulator chamber below said second packer, said production and vent tubing extending in a sealed way through both packers and into the accumulator zone.
In U.S. Pat. No. 3,797,968 to Elfarr, issued Mar. 19, 1974 APPARATUS FOR FLOWING LIQUID FROM A WELL, describes a downhole siphon pump apparatus for producing oil from an oil well where formation pressure is insufficient to move oil from bottom-hole to surface, powered by compressed gas from surface. The Elfarr system is comprised of two concentrically deployed tubing stings inserted within the well""s casing. The inner tubing string for its length to near bottom carries compressed gas from surface. The annulus between the two tubing strings carries oil from the bottom-hole pump to surface. Near the bottom, the conduits cross-over via cross-over passages in a fitting. At various heights in the production annulus, check valves are deployed to permit upward-only flow of oil. At bottom, a check valve permits oil from the formation in the casing to enter the outer conduit and accumulate in the tubing string""s outer annulus. A check valve in the bottom of the inner tubing string, below cross-over and above the lower check valve is deployed permitting oil to flow upward only into the production passageway for pumping. The inner tubing at surface can be pressurized, causing the pressure differential across the check valves at bottom to (a) close communication between the two tubing strings and the casing; and (b) open communication between inner and outer conduits, permitting and causing the accumulated captured oil to flow upward toward the surface. The formation is thus not exposed to higher than bottom-hole ambient pressure. The check valves in the outer production annulus and the system can be cleared by removal of the inner tubing string, which is for most of its length (from surface to cross-over) used only to carry pressurized gas from the surface.
There is thus formed a pump with two chambers deployed up-hole, formed of concentric-tubing strings, cross-over fittings, and check valves, with the removable inner string sitting on nipples and seats on the inner surface of the outer tubing string, for pumping oil from wells using introduced pressurized gas from surface. The pump was designed to move oil to surface in an oil well to enhance production, but it is complex, small-diameter, and requires specialty fittings, seats and nipples, and it has a large number of complex and special-purpose valves. It would interfere significantly with free gas-flow, and while it operates on pressure differentials (a siphon-like function), it is for the specific purpose of pumping oil to surface.
U.S. Pat. No. 4,509,599 to Chenoweth et al, GAS WELL LIQUID REMOVAL SYSTEM AND PROCESS, issued Apr. 9, 1985, describes a system and apparatus for dewatering a gas well which, at the well-bore""s bottom end has collected sufficient fluid (water, for example) that the bottom-hole pressure (adjacent the production from formation to casing) is insufficient to independently transport gas from formation to surface (through, it is assumed, a conduit or tubing string secondary to the casing""s cross-section).
Chenoweth""s invention provides for the division of the casing""s annulus into two independent conduits (typically, a tubing string and the casing/tubing annulus), with a compressor at surface producing lower pressure within one of the conduits sufficient that production pressure (from formation) at bottom-hole propels gas-fluid mixture to surface through that low-pressure conduit, permitting gas alone to flow through the other conduit (once the over-pressure situation has been remedied by removal of sufficient fluid from the well""s bottom). There is also provided a means of heating the top segment of the mixed fluid-gas-carrying conduit to avoid precipitation of paraffin.
It is a necessary part of the Chenoweth invention that both conduits communicate with formation throughout the process claimed. It is also provided that a lowered pressure (partial vacuum) is applied, at surface, which can be difficult, and which might damage the well""s production interface. It does not contemplate any need to segregate production zones from the method or apparatus for dewatering the well. It is a xe2x80x9cclosedxe2x80x9d system in that all matter exhausted from the wellbore is (once liquids are separated) inserted into the gas collection system to the sales line.
The discussion in the prior art cited in the Chenoweth specification discloses the insertion of highly pressurized gas into a well-bore to cause the evacuation of gas and liquid from well-bottom through a second xe2x80x9csiphon tubexe2x80x9d string to xe2x80x9cunloadxe2x80x9d a gas well, as well as methods involving gas-lift valves, downhole mechanical pumps, differential gas intermitter systems, and the like, and claims to overcome the difficulties inherent in those systems. Of particular interest here is the mention of insertion of high pressure gas to xe2x80x9cblowxe2x80x9d liquid-gas mixtures up a siphon tube depended within the well""s casing, as a variant to reduction of pressure from collection system pressure ranges to lower, atmospheric pressure ranges, in order to exhaust gas through the siphon tube at high enough velocities to unload the problematic well.
The cure in the objects of the Chenoweth ""599 patent for damage potential from highly pressurizing the casing (and production formation) is to significantly reduce the pressure through the siphon tubing, thus permitting formation pressures to be the highest pressures in the system (and thus eliminating formation damage potentials caused by insertion of high pressure gas from surface into casing, which can xe2x80x9cdrivexe2x80x9d entrapped liquids and solids at bottom-hole in the well-bore back through the casing perforations into formation).
U.S. Pat. No. 4,437,514 to Canalizo DEWATERING APPARATUS, issued Mar. 20, 1984, discloses a valve-set comprised of two valves, one permitting liquid-only fluid flow from casing-tubing annulus into tubing, the other, higher, valve permitting communication from casing-tubing annulus of gas or liquid into the tubing. The valve-set is put at the lower end of a tubing string, and may be actuated automatically or from surface.
During operation, when the well-bore begins to fill with water from production, the water rises within the casing-tubing annulus, and when its hydrostatic pressure is sufficient to open the lower valve, water is made to enter the tubing string. When the tubing string is filled to a desired level, the second, higher, valve is opened, and the tubing, being open to atmosphere at surface, is over pressured by pressure of produced gas and liquids, which thus enter the bottom of the tubing string and cause a gas bubble to enter and to evacuate water and gas to surface, optionally with the use of a pig or plunger between the gas and trapped water. This may be referred to as a gas-lift intermitter.
When suitably evacuated, the pressure in the casing-tubing annulus will have dropped, it being sealed at surface with collection tubing with a third check (one-way out) valve, which causes the second, higher valve to close, shutting off produced pressure to the tubing.
There is no need to insert additional pressure to the casing-tubing annulus, nor is any method disclosed. There is no seal of the well-bore above production, as there is no introduced pressure to damage the producing formation. A problem which the ""514 system will encounter in deep wells is that the longer length tubing introduces more chances of the equipment binding in the tubing string.
U.S. Pat. No. 4,226,284 to Evans, GAS WELL DEWATERING METHOD AND SYSTEM, issued Oct. 7, 1980, provides for a closed system of pipes and valves operable via a timer, which permits casing pressure to dewater a gas well by blowing produced gas down an inserted tubing string using pipeline or formation pressure, so that entrained water with produced gas is blown back up the casing/tubing annulus directly into a conventional production collection system (which typically includes a normal gas flow line and liquid separator means), in order not to waste produced gas in the dewatering process.
Otherwise, the Evans invention shows no increase in casing pressure, nor any method of protecting the formation, the inventive step being the closed piping and valve system with timer. Energy is provided again from pipeline or production pressure to the lift mechanism.
Canadian Patent No. 848,766 to Kelley and Kelley, LIQUID CONTROL FOR GAS WELLS, issued Aug. 11, 1970, discloses an apparatus to control the liquid depth in gas and oil wells. This invention is comprised of a positive action liquid/gas separator within a well-bore, a tubing string within the well-bore connected to the separator, a gas lift valve connected to the tubing string and responsive to predetermined pressure (supplying gas under pressure from the well-bore to the tubing string so it lifts liquid through the tubing string to the surface), and a free piston member (or pig or swab) retained within the tubing string for movement between the gas lift valve and the surface below the column of liquid (to assist the gas lift in driving the column of liquid from the well by segregating the water above the lift gas).
Canadian Patent No. 890,226 to Kelley and Kelley, APPARATUS FOR REMOVING LIQUID GAS FROM AND OIL WELLS, issued Jan. 11, 1972, discloses an apparatus for the removal of liquid from gas and oil wells. This invention is similar to Canadian Patent No 848,766 absent the liquid-gas separator but with the addition of a self-lubricating free piston member (to replace the free piston member). The invention shows an intermitter gas lift with a free piston. The free piston is disposed in the tubing string and retained by a bottom stop and catcher apparatus, and is lifted by introduced gas from formation below the column of liquid to distribute the pressure of gas admitted to the tubing string by the gas lift device across the bottom of the column of liquid and to avoid the gas dissolving in the liquid as it is lifted to lower pressure regions. This invention provides automatically regulated intermittent flow through the tubing string without manual control or cycle timers. This invention and Canadian Patent No. 848,766 require relatively high inherent (not introduced) differential gas pressures to operate reliably.
U.S. Pat. No. 5,339,905 to Dowker, GAS INJECTION DEWATERING PROCESS AND APPARATUS, filed November 1992, discloses a gas injection dewatering process and apparatus. In this invention a conduit is provided in a watered-in well in the form of tubing of smaller diameter than the well-bore or cased bore to conduct water from the bottom of said well-bore to the surface, said conduit including a check check-valve such that when water flows upward, it cannot then reflux backward.
Periodically, a volume of dried, pressurized natural gas is injected into the lower end of the conduit from an adjacent gas line from surface and which injected gas is then allowed to expand, thereby forcing a slug or column of water upward through the conduit toward the upper end. This is a typical xe2x80x9cgas liftxe2x80x9d method.
This xe2x80x9cpulsexe2x80x9d of induced gas is repeated, being pumped periodically down the secondary conduit through the well-bore or casing through that conduit, and then being allowed to expand within the production tubing conduit in order to cause a pulse of increased pressure within the production conduit which is meant to cause the water into which that conduit is depended to be forced to surface.
In one embodiment, there are two conduits deployed essentially in parallel down the well-bore""s length from surface to below water, with one being an exhaust conduit and the other being a delivery system for the pressurized dehydrated natural gas, where the said dehydrated natural gas is injected into the body of the exhaust conduit, thus causing a xe2x80x9cbubblexe2x80x9d of expanding gas to flow upwardly within the said exhaust conduit, decreasing pressure within the exhaust conduit and thus pulling water up the exhaust conduit coupled with pulses of expanding gas.
There are a number of difficulties with this system, chief amongst them being that by pressurizing the well-bore to force water up the conduit this method also can cause the pressure within the formation with which it is in communication to increase, and incidentally causing the water (and any included matter) accumulated in the well-bore to be forced back into the formation, together with any sand or other substance in the well""s bore at bottom-hole near the casing, and thus xe2x80x9creloadingxe2x80x9d and potentially damaging the formation, rather than evacuating the accumulated water at bottom hole.
Additionally, this invention requires special equipment to provide dehydrated natural gas under pressure, and requires the deployment of specialized dual/parallel tubing and injector mechanisms, thus being more costly than desired.
U.S. Pat. No. 4,823,880 to Klatt, GASWELL DEHYDRATOR VALVE, filed September 1988, discloses a gas well dehydrator valve. This invention deals with the particular situation of two contiguous producing gas zones within one well-bore, both being produced simultaneously, the lower one through a tubing dependent past a segregation packer in the well-bore between the two zones. When the upper producing zone produces sufficient water such that the hydrostatic pressure caused by that water""s accumulation above the segregation packer within the well-bore overloads the production from that upper zone, this invention""s system provides for a special valve within the packer to allow communication of the gas under higher pressure from the lower formation to be introduced to the annulus between the inner conduit and the well""s casing, where the upper production is done and where there is now water accumulated, in order to use that introduced gas pressure to essentially xe2x80x9cpumpxe2x80x9d or xe2x80x9cblowxe2x80x9d said accumulated water up that annulus to surface, reducing the hydrostatic pressure and unloading the upper formation, allowing production from the upper reservoir to resume under natural pressure (when the specialized packer-valve is reset).
This invention has a number of particular deficiencies, notably: the requirement for two contiguous production zones, the lower zone not accumulating water from formation (i.e. maintaining its natural pressurization sufficient to clear the accumulated water in the upper zone); the requirement for specialized and complex valve and actuation devices at the segregation packer and the packer itself, together with difficulties inherent in properly locating and sealing those apparatus properly in the well-bore.
U.S. Pat. No. 4,171,016 to Kempton, WATER REMOVAL SYSTEM FOR GAS WELLS, filed February 1978, discloses a water removal system for gas wells. This invention involves a set of concentrically deposed tubes with a specialized injector at the bottom end within a well-bore, depending into water at the well-bore""s bottom. Pressurized water is pumped down the annulus between the inner tube""s outer wall and the outer tube""s inner wall, and is injected at bottom upwardly into the inner tube""s annulus, causing said water to jet under significant pressure up the inner tube, which in turn causes the pressure within the inner tube to drop somewhat from the pressure within the formation, and thus causing the water within the well-bore to flow or be thus pumped to surface. This is a down-hole injection or jet pump.
There are a number of difficulties with this system and method, chief among those being the requirement to pump large volumes of water at relatively high pressures into the well-bore near the formation in order to cause sufficient jet-pumping pressure differentials to evacuate the water from near to the formation. Additionally, if the injector becomes damaged or clogged, this system will result in additional volumes of water being introduced to the production zone of the well under high pressure, thus potentially seriously damaging that well""s future ability to produce gas.
U.S. Pat. No. 4,596,516 to Scott et al, GAS LIFT APPARATUS HAVING CONDITION RESPONSIVE GAS INLET VALVE, issued Jun. 24, 1986, discloses a gas lift where a siphon tube is deployed within the well""s casing, and near the bottom end of the siphon tube is a valve, operable from surface (or alternatively, responsive to the differential between hydrostatic pressure of a column of water within said annulus and the pressure within the siphon tubing), which, when opened, permits communication from the casing""s annulus outside of the siphon tubing with the interior of the siphon tubing.
The valve structure is sealed to the casing (below the communications openings) by a set of two packings, situated above the perforations from formation to the casing""s annulus. The seals or packings are provided with a pipe which (full-time) communicates from formation to the casing""s annulus, said pipe extending within said annulus upward from the packing to above the operative parts of said valve.
When the wellbore including the casing annulus and the tubing string (siphon) are filled with produced water, and it seems desirable to evacuate said water, the valve within the tubing is opened, permitting produced gas from formation to enter said valve above the packing seals, and to enter the siphon tubing, causing a bubble of produced gas to xe2x80x9cburpxe2x80x9d up the tubing, and evacuate water therefrom to surface (with the optional assistance of a piston-like xe2x80x9cpigxe2x80x9d which travels above the gas slug but below the moved liquid to surface, where the liquid is drained off and the pig is permitted to descend the tubing to bottom).
There is provided no means of fully sealing the wellbore just above casing perforation to protect the production formation during pressurizing of the well. Formation pressure, when introduced through the novel valve/seal means, is sufficient to provide gas-lift to evacuate water without adding or inserting pressure to casing.
This, then, is a typical downhole valve system of evacuating water from a producing gas well""s bore using a second tubing string and production pressures and gases to provide necessary lift.
It is an object of this invention to overcome or mitigate as many of the difficulties apparent in the prior art as is workable.
This invention is a system and apparatus for the removal of water from a gas well.
The present invention overcomes or mitigates some shortfalls in the prior art, chiefly those which concern potential damage to producing formation or to the wellbore and casing from injected pressurized fluid or gas to power pump or lift devices, the insertion of pressure into the well-bore which could communicate either directly or via pressure conveyed through fluid and gas in the well-bore to the production formation, or similar problems. As well, the present invention provides an efficient system of dewatering and thus unloading a watered-in gas well with locally obtained pressurized gas using facilities, materials and equipment which are conventionally available and easy to operate either manually or automatically in the field.
In accordance with a broad aspect of the present invention, there is provided an assembly for production of gas from a down-hole gas-producing formation to surface through a well bore lined with casing having openings through which gas can pass from the formation into the casing and the assembly being capable of periodically clearing of accumulated liquids when the pressure of produced gas is insufficient to overcome hydrostatic pressure of the accumulated liquids, the assembly comprising: a tubing string extending from surface through the casing and having a lower open end positioned below a point at which the hydrostatic pressure of accumulated liquids exceeds the formation pressure of produced gas, the tubing string having a fore sealed from fluid flow communication with the casing except through the lower open end; a means for collecting fluids passing from the casing at surface; a means for collecting fluids passing from the tubing string at surface; a check valve positioned in the casing above the openings and below the lower open end of the tubing string, the check valve permitting flow of fluids from formation through the openings to the lower end of the tubing string, but restricting reverse flow therethrough; and a means for introducing gas from a source of pressurized gas at surface to either the casing or the tubing string to create sufficiently high pressure above the accumulated liquids to push the accumulated liquids from bottom-hole up through the other of casing or the tubing string to surface and out of said well.
In accordance with another broad aspect of the present invention, there is provided a method of unloading accumulated water from a well bore to a level sufficient to permit gas to be produced from the well bore, using formation pressure, the well bore being lined with casing and having openings in the casing to permit produced gas to pass from a formation into the casing, the method comprising: providing an assembly including a tubing string extending from surface through the casing and having a lower open end positioned below a point at which the hydrostatic pressure of the accumulated liquids exceeds the pressure of produced gas, the tubing string having a bore sealed from fluid flow communication with the casing except through the lower open end; a means for collecting fluids passing from the casing at surface; a means for collecting fluids passing from the tubing string at surface; a check valve positioned in the casing below the openings and above the lower open end of the tubing string, the check valve permitting flow of fluids from the openings or perforations to the lower open end of the tubing string, but restricting the reverse flow therethrough; and a means for introducing gas from a source of pressurized gas at surface to either the casing or the tubing string; closing the check valve; introducing gas from surface to either the casing or the tubing string to create sufficiently high pressures above the accumulated liquids to push the accumulated liquids from bottom-hole up through the other of casing or tubing string to surface and out of said well bore; and stopping introduction of gas from surface to permit the well bore to return to production of gas from the formation.
The casing lines the well bore, as is known and can be standard casing or any other well bore liner. The well bore can be vertical or horizontal and can be the main well bore or laterals kicked off from a main bore hole. Where the well bore services more than one production zone, has a large internal volume or includes numerous lateral well bores, packers can be used to isolate areas into which gas is introduced to apply pressure above accumulated liquids.
In some situations, such as in a dangerous or sour gas well, or other regulated wells, or where the integrity of the casing is suspect, production may be through an internal conduit or tubing string depended or placed within the said well""s original casing. The internal conduit extends downward from surface to a packer above the production formation perforations, the annulus between the casing and the internal conduit being filled with an essentially inert fluid or other substance to protect the adjacent environment from leaks in the well""s casing. In such cases, the internal conduit would be considered for the purposes of this invention to be the xe2x80x9ccasingxe2x80x9d as described in this disclosure.
The openings in the casing will generally be perforations formed through the casing and cement behind the casing, but can also be other arrangements for permitting gas to flow from the formation into the casing such as, for example, a slotted liner or screen or open hole.
The tubing string can be an existing production or production-enhancement tubing string or a string run in for the purpose of unloading accumulated water from the well. As such, the tubing can be of any desired diameter, provided fluid flow through the casing about the tubing string and through the tubing string is significantly restricted. The tubing used in the tubing string must be rigid enough so that it will not collapse due to the pressure differentials which will be present on either side of its walls.
In conventional production wells, tubing string diameters can be of various sizes and still be used in this invention. Typical diameters are xc2xdxe2x80x3, xc2xexe2x80x3, 1xe2x80x3, 1xc2xcxe2x80x3, 1xc2xdxe2x80x3, etc. The only requirement to operate this invention is that the tubing string""s bore (if it is used as the exhaust conduit) has to be of a size that the liquids being removed do not flow down past the gas flowing up the conduit (for example, 3xc2xdxe2x80x3 outside diameter tubing is too large if installed in a 4xe2x80x3 inside diameter casing, as the compressed gas bubbles through the liquid, the liquid flows down past the gas, and in both instances the liquid is not produced to surface to unload the well).
The present invention will work with xc2xdxe2x80x3 to 2xe2x80x3 tubing installed in a 4xc2xdxe2x80x3 casing. Generally, it is preferred to use the tubing which is already present in most producing gas wells susceptible to becoming overloaded with water. In cases where the tubing was too large, the present invention generally works if the roles of the two conduits are reversed.
Since the level of accumulated liquids will be brought down only to the level of the lower open end of the tubing string, the lower open end of the tubing string must be positioned below a point at which the hydrostatic pressure of the accumulated liquids exceeds the pressure produced gas. Preferably, the lower end of the tubing string is positioned closely above the openings so that a maximum amount of accumulated liquids can be unloaded in each operation.
The bore of the tubing string can be sealed from communication with the casing by various configurations. One useful configuration is a seal positioned in the annulus between the casing inner wall and the exterior surface of the tubing string at the well head.
The check valve can be mounted permanently or temporarily within the casing. The check valve should be of type suitable for the nature of the well bore and casing. For example, in a vertical cased well, a check valve can be a ball-and-seat mechanism well known in the art, while in a horizontal well, the check valve can be a sprung flapper valve or similar mechanism.
The check valve can be installed either by wireline or can be attached to the lower end of the introduced tubing string, depending upon which conduit is acting as the xe2x80x98casingxe2x80x99. If the tubing (which is acting as the casing, referred to as the xe2x80x9ccasing/tubingxe2x80x9d) has a packer isolating the casing/tubing from the well""s annulus, the check valve can be installed with conventional wire-line equipment and techniques, and set at the lowest point in the casing/tubing (which is the acting casing) by setting the check valve at the lowest connection or bottom end in the casing/tubing. The (typically, coiled) inserted second tubing string would then be xe2x80x9clandedxe2x80x9d as close to the check valve as practical, but above the check valve which had been previously installed by wire-line within the casing/tubing at its lowest connection, above its bottom, and below the inserted tubing string.
In sweet wells, it is generally not necessary to pack-off or isolate the annulus from the casing/tubing by a packer, and in those wells, coiled tubing is used to conduct accumulated water from the well-bore to the surface, the coiled tubing carrying with it as an installed piece at its bottom, an assembly consisting of a landing cone attached to the bottom of the check valve, which in turn is attached to a slotted sleeve or similar fitting (to permit communication between the interior of the coiled tubing and the tubing-to-casing annulus through the slots, above the check valve) which is in turn attached to the lower end of the coiled tubing; a conventional slip-stop removable packer/seal (ordinarily used to seal the annulus between a fitting on the coil tubing""s bottom end""s circumference and the casing/tubing within which the coiled tubing is sought to be sealed) is first installed in the casing/tubing above the openings to the production formation, and the coiled tubing with its bottom assembly is lowered until the weight of the coiled tubing and its assembly rests, pressing its bottom-most landing cone into the slip stop, causing a secure seal of the casing/tubing annulus to the check valve, isolating (when the check valve is closed) the two conduits (coiled tubing""s bore plus casing/tubing-to-coil tubing annulus) from the production openings.
The wire-line and coiled tubing installers are equipped with counters which indicate the length of wire or tubing inserted into any well, which indicates the depth to which the wire-line or tubing has been lowered. Those counters are generally sufficiently accurate for placement of the check valve above the openings to formation. If the check valve is lowered below those openings, the well-bore will not hold a test pressurization, in which case the check valve should be moved up (usually not more than a meter or so at a time), and retried.
If the seal of the check valve to the casing/tubing is effected by lowering the coiled tubing to rest on a slip stop packer, for example, care should be taken in the temperature of the pressurized gas which is inserted into the coiled tubing""s bore to unload the well, because it has been found that insertion of chilled air (from very cold surface conditions, for example) may cause the coiled tubing to contract longitudinally enough to lift the check valve and seat assembly at its bottom off the slip stop, and to thus permit communication between the siphon arrangement above the check valve and the openings below the check valve, thus defeating the purpose of the invention.
The means of collecting fluids passing from the casing and the tubing string at surface can be a system of conduits, seals and valves and preferably includes a gas collection system including gas/liquid separators, compressors and/or containment vessels as is known in the art.
The seals between the two conduits (for example, coiled tubing inserted into a cased well) are usually accomplished using conventional coiled tubing hangers, an example of which is produced by Select Energy Systems, Inc. of Calgary, Alberta. Such hanger systems provide a composite bag through which the tubing is led, and which when compressed forms a very tight seal between the hanger""s body and the tubing""s exterior surface.
The means for introducing gas preferably includes conduits, seals and valves from the source of pressurized gas. In one embodiment, the source of pressurized gas is a high pressure producing gas well. In another embodiment, the source of pressurized gas is a compressor connected to the well head by a conduit. In that case, a valve is preferably provided to permit closing off the conduit to the compressor or other source of pressurized gas. It will be apparent to those skilled in the art the compressor should preferably be automatically operable without requiring attention or priming. It is desirable that the compressor be capable of extended unattended stop-and-start operation. One type of suitable compressor is very similar to the low to moderate volume but moderate to high pressure output requirements met by compressors used in filling scuba-diving tanks. The higher the volume capacity of the compressor the faster the water is evacuated. The compressor can also be of a type mounted on a truck or large vehicle. Where there is a plurality of production wells closely positioned, there can be one permanent or semi-permanent compressor or source of high pressure gas placed to service all of these wells.
The pressurized gas should preferably remain in a gaseous state at the pressures to be used. As an example, the introduced gas should not be something like pure propane, which in most circumstances at the pressures required, would be compressed into its liquid, on-gaseous state. Suitable gases for introduction are, for example, surface air or compressed natural gas from the collection pipeline at the well head or produced at a nearby well, or nitrogen or other inert gas where chemical reaction might be problematic.
Where the well is producing sweet gas at low pressures, it has been preferred to use air from atmosphere through a compressor as the pressurized fluid in the present invention. Where a well is near a source of natural gas under pressure higher than that required to unload the well and in sufficient volumes, such as a nearby producing well or pipeline, those sources can be used as the surface source of pressurized gas for insertion. Where a well is near to a source of natural gas of sufficient volume but insufficient pressure to be utilized without compression, compression may be used with such a gas source.
Ideally, the water produced by the unloading process will be produced into the collection system of separators and sales lines, and carried away from the well-site for proper disposal. The compressor used should be large enough to provide adequate volumes of pressurized gas in the required pressure ranges to produce the accumulated liquids within a reasonable period of time. Those types of compressors will be apparent to one skilled in the art. Generally, the less time spent unloading, the more time spent producing, the better.
It is to be understood that the pressure-containing capacity of well components such as well-heads, conduit, piping, joints, packers, fittings, compressors, valves, and the like, will have to be sufficiently higher than the pressures encountered during the operation of the system. It is also to be understood that all components of the assembly according to the present invention, must be suitable for use in well bore conditions, which may be corrosive, etc.
The pressurized gas can be introduced to either the casing or to the tubing string to act on the accumulated liquids. In a preferred embodiment, the gas is introduced into the casing causing liquids to be forced up through the tubing string to surface. This is preferred where the casing is large diameter and the tubing is of relatively small diameter. It is generally preferred to produce pressured gas into the conduit with the smaller cross-section, and to exhaust the liquids from the conduit with the larger cross-section.
The check valve can be closed, in one embodiment, when hydrostatic pressure caused by the accumulation of water from the production zone in the bottom of the casing above the check valve exceeds the pressure from the formation. In another embodiment, the check valve is forced to close by introducing gas from surface to increase the pressure above the check valve.
The introduction of gas from surface to force liquid out of the well can be initiated when it is determined that the check valve is closed, as will be indicated by observing the flow or produced gas from the well. When the flow of produced gas ceases, the check valve is assumed to be closed. Alternatively, the introduction of gas from surface can be initiated on a regular basis by use, for example, of a timer.
Prior to introduction of gas, the casing will be sealed to prevent escape of introduced gas and produced fluids, in order to utilize the pressure differentials, except through the conduit being used to exhaust the accumulated fluids.
The introduction of gas from surface can be continued until the introduced gas begins to be expelled after the accumulated liquids are exhausted, or for a selected period of time, regardless of whether water continues to be evacuated.
There are other situations and well configurations which those skilled in the art would consider to be equivalent to xe2x80x9ccasingsxe2x80x9d or xe2x80x9cconduitsxe2x80x9d or xe2x80x9ctubing stringsxe2x80x9d, or to xe2x80x9cboresxe2x80x9d or xe2x80x9csurfacexe2x80x9d or xe2x80x9ccollection systemsxe2x80x9d or xe2x80x9cgasxe2x80x9d, xe2x80x9cfluidxe2x80x9d, xe2x80x9cvalvexe2x80x9d, and the like, and such terms are to be read expansively rather than restrictively, the description here being made to inform those skilled in the art of the concept of the invention and some of its embodiments, but not so as to restrict the claims set out herein.